Mould allergens are recognised as a major cause of allergic diseases in humans and animals, including asthma, allergic rhinitis, allergic conjunctivitis and allergic dermatitis. In colder climates, moulds can be found in the outdoor air starting in the late winter, and peaking in the late summer to early fall months (July to October). In warmer climates, mould spores may be found throughout the year, with the highest levels found in the late summer to early fall months. While indoor moulds can occur year round and are dependent on moisture levels in the home, indoor mould levels are higher when outdoor mould levels are higher. Therefore, a common source of indoor mould is from the outside environment, although can also be from indoor mould contamination.
There are thousands of types of mould; however, only a few of these are commonly associated with allergy. The following are the most likely causes of allergic disease based on the types of mould spores collected in the air: Alternaria, Cladosporium, Aspergillus, Penicillium, Helminthosporum, Epicoccum, Fusarium, Aureobasidium, Phoma, Rhizopus, Mucor, Smuts and Yeasts. Moulds in the genus Alternaria, in particular Alternaria. alternata, and the genus Cladosporium are considered to be among the most important allergenic fungi.
Cladosporium is the most common airborne outdoor mould. Alternaria is one of the main allergens affecting children. In temperate climates, airborne Alternaria spores are detectable from for most of the year (typically May to November in the northern hemisphere), with peaks in late summer and autumn. Dispersion of Alternaria spores occurs during dry periods. These feature higher wind velocity and lower relative humidity, which result in peak dispersion during sunny afternoon periods
Although considered to be an outdoor mould, Alternaria will grow anywhere that provides sufficient moisture and a suitable growth substrate. Accordingly, Alternaria is commonly found indoors, in particular in damp areas such as basements, kitchens or bathrooms. Alternaria is commonly found in refrigerator drip trays, air conditioners, waste containers, mattresses, foam rubber pillows, or even in condensation on windows. It is one of the most common mould spores found in house dust in both North America and Europe. It is effectively impossible to avoid Alternaria allergens.